Resources on Roman numeral calculators, "computers," puzzles, and converters.
Computer Romanus Edward Hobbs has developed a "computer" to convert Arabic numerals into Roman numerals. You can also learn how to convert dates using Roman numerals.
Numerarium "Based on a popular European TV game show, Numerarium challenges you to build a given number by adding, subtracting, multiplying and dividing other numbers. Trick is... the arithmetic is in those cryptic Roman numerals!"